Your server should have TWO network ranges - a public network and a private network. From your post it sounds like you have 6 public addresses for your network.
You should create a second network for your private network and define a range of IP addresses in this network for the VPN client range. That network should use private class addresses such as 192.168.x.x or 10.x.x.x, but you cant use them until you have an interface on the server in that range.
The "Unknown Network In Range" message looks like a bug to me. I've set up several Lion servers for VPN and I get this message every single time, no matter what address range you supply.
If you know that the addresses you have put in the VPN range are correct for your network, just ignore the message and the VPN will work correctly.